If I press Shift + D to duplicate an object will it always be just one copy?
A duplicate will be created every time you press Shift + D and right or left click the mouse button.

If you only press Shift+D multiple times, but only click the mouse once, only one duplicate will be made.

If you're in Object Mode, then you can just right click each one as you said.

If you're in Edit Mode then right click to select one of the vertices and press Ctrl + L.  This will select all the geometry connected to that vertex.  Since none of the duplicates are connected, it will select each duplicated instance: